_ The long-term goals of this proposal are to determine the neurobiological mechanisms that underlie sex differences in the function of the hypothalamo-pituitary-adrenal (HPA) axis. In humans and animals, sex differences in HPA reactivity are well established; females exhibit a more robust activation of the HPA axis following stress than do males. Our hypothesis is that these sex differences in adult hormonal stress responses are largely the result of the opposing actions of testosterone (T) and estrogen (E) on HPA function. Activation of the HPA axis is a basic response of animals to environmental perturbations that threaten homeostasis. Neuroendocrine stress responses are controlled by neurons residing in the paraventricular n. of the hypothalamus (PVN). These neurons synthesize and secrete corticotropin-releaslng hormone (CRH). Other neuropeptides, such as vasopressin (AVP) and oxytocin (OXY) are also synthesized by PVN neurons and act to modulate the actions of CRH. Thus, neurons in Lhe PVN function as an integratory node, coordinating both excitatory and inhibitory inputs to produce a tightly controlled output. E and T have also been shown to modulate stress responses. In females, E enhances stress activated ACTH and CORT secretion, in part through direct action on neuropeptide neurons in the PVN. In males, T decreases the gain of the HPA axis. Although androgen receptors are NOT found in neuroendocrine neurons of the PVN, our data show that androgens can inhibit PVN neuron function through a novel pathway. Accordingly, within the male PVN, T may be metabolized to 5c_-androstane-36,1713-diol (3_-diol)which can bind ER_ and potently inhibit the activity of PVN neurons. In this application we will address the possibility that ER-13 is a dual function receptor which can act to both activate and inhibit neuropeptide components of the HPA depending on the available ligand. This switch also depends on the splice variant of ERI3 expressed. To address this, we will determine 1 ) the splice variant population and their hormone regulation in neuropeptide neurons of the PVN. 2) The interactions of ERI3 variants with E and 313-diol in the regulation of the CRH promoter. 3) We will determine if appropriate androgen metabolizing enzymes are found in PVN neurons and 4) we will determine if this pathway acts through ERI3 to regulate HPA function. Studies described in this application will focus on the function of ER-13 in PVN neurons, however, they will also provide information necessary for directing future studies examining the neural circuitry regulating HPA reactivity.